The invention is based on a device for optical distance measurement as generically defined by the preamble to the independent claim.
Optical distance measuring devices per se have long been known and by now are also sold commercially. These devices emit a modulated beam of light, which is aimed at the surface of a desired target object whose distance from the device is to be ascertained. The light reflected or scattered by the target area aimed at is partly detected again by the device and used to ascertain the distance sought.
The range of use of such distance measuring devices generally encompasses distances in the range from a few centimeters to several hundred meters.
Depending on the distances and the reflectivity of the target object, different demands are made of the light source, the quality of the measuring beam, and the detector.
The optical distance measuring devices known from the prior art can be divided in principle into two categories, depending on the disposition of the transmission and reception channels necessarily present in the device. First, there are devices in which the emission channel is disposed at a certain spacing from the reception channel, so that the respective optical axes extend parallel to one another. Second, there are monoaxial measuring devices, in which the reception channel extends coaxially to the emission channel.
The biaxial measuring systems have the advantage of not requiring complicated beam splitting for selecting the returning measurement signal, so that optical crosstalk from the emission channel directly into the reception channel can for instance better suppressed.
On the other hand, in biaxial distance measuring devices there is the disadvantage among others that for the range of relatively short measurement distances, parallax can cause detection problems:
The projection of the target object onto the detector surface of the measurement receiver integrated with the device, which projection, for long target distances, is still unequivocally located on the detector wanders increasingly away from the optical axis of the receiving branch as the measuring distance becomes shorter and furthermore undergoes a variation in the beam cross section in the plane of the detector.
This means that unless further provisions are made in the device for the near range of detection, that is, for a short distance between the target object and the measuring device, the measurement signal can tend toward zero.
From German Patent Disclosure DE 43 16 348 A1, a device for distance measurement is known with a visible measuring beam generated by a semiconductor laser; its receiver contains an optical waveguide with an optoelectronic converter downstream of it. The light entry face into the fiber of the waveguide is disposed in the projection plane of the receiving lens element of this device for great object distances and from this position can be shifted transversely to the optical axis.
In this way, in the device of DE 43 16 348 A1, the measuring beams, which for short object distances strike the receiving lens element more and more obliquely, can be directed, via the tracking of the optical fibers, onto the light-sensitive surface of the detector, for a detector that is not three-dimensionally variable.
The requisite electronic triggering of the tracking and the use of additional and in particular also moving parts in the distance measuring device disclosed in DE 43 16 348 A1 mean a not inconsiderable expenditure, which increases the complexity and thus the costs and vulnerability of such a system.
Alternatively, DE 43 16 348 A1, for solving the parallax problem in biaxial measuring devices, proposes that the optical waveguide entry face be stationary, and by optical deflection means in the peripheral region of the receiving lens element to assure that the measuring light beams can still strike the detector even as the distance from the object is decreasing. Among other things, it is proposed that a deflection mirror be used for this purpose, which deflects the measuring beams, entering the measuring device from a short object distance, onto the detector. For solving the same object, the same reference also proposes the use of a prism, which is placed in the peripheral region of the receiving lens.
The necessary additional components must be considered a disadvantage in solving the problem in the above way. Moreover, a negative interaction of these additional components with the beam path of the measuring beams from a great distance cannot always be precluded, so that for this reason as well, signal impairments can occur that restrict the usable measurement range of the distance measuring device.
The device for optical distance measurement of the invention having the characteristics of the independent claim has the advantage over the prior art of being able to dispense with additional optical elements for correcting the parallax problem, and nevertheless of making a measurement signal on the detector possible that is also sufficient for the near range.
The shape of the light-sensitive, active face of the detector of the invention is selected such that even in the near range, a signal of adequate amplitude is available at the detector surface.
This makes it easily and reliably possible to expand the measurement range accessible to this measuring device.
Compared to the devices for optical distance measurement known from the prior art, the device of the invention has the advantage that the distance covered by the optical beam is not affected by the means for overcoming the parallax problem, so that these means have no negative effects on the distance measurement.
Moreover, no calibration of additional and in particular moving components in the measuring device is needed.
Advantageous versions of the device of the invention will become apparent from the characteristics recited in the dependent claims.
Advantageously, the size of the light-sensitive face of the detector of the receiver unit is selected to be so great that a still-adequate signal strikes the detector even in the near range. Because the measuring beam returning from the target object, for a decreasing object distance, migrates laterally in the common plane of the optical axis of the emitter unit and the optical axis of the receiver unit, the detector will advantageously assume a form that is elongated in that direction. In this way, the dependency of the direction of the returning measurement signal on the distance of the measuring device from the target object is taken into account by the concrete shape, according to the invention, of the effective, active detector face.
The shape according to the invention of the effective detector face furthermore makes it possible to take into account the dependency of the intensity of the returning measurement signal on the distance of the measuring device from the target object. Because the square law is fundamental to the change in intensity as a function of the distance traveled, the returning measurement signal for the near range is markedly larger than for target objects that are located far away from the measuring device.
The length of the effective detector face perpendicular to the common plane of the optical axes of the emitter unit and receiver unit can therefore decrease to the extent that the light signal, because of the shorter distance, increases in the near range. This has the advantage that because of the length of the detector, enough light from the near range will still strike the detector, yet because its active, light-sensitive face is becoming smaller in this direction, the detector cannot be oversteered by the light from the near range. A displacement of the detector out of the focus of the receiving lens along the optical receiving axis for adapting the signal intensity striking the detector is thus no longer necessary in the device of the invention.
The embodiment of the detector face according to the invention thus also has the advantage that the ratio of useful light to extraneous light is improved markedly, so that for this reason as well, the measurement accuracy of the device in the immediate near range is enhanced, and thus the measurement range of the device is expanded.
In terms of the size of the face of the detector, it must merely be assured that the effective area in the region of the detector in which light from far-away target objects strikes the detector surface be large enough to detect as much as possible the entire signal. This is another consequence of the square law, to which the detected intensity is subject, and leads to a relatively weak detection signal for far-away measurement objects.
The lateral length of the detector must accordingly be so great that enough light from the immediate near range of detection will still reach the detector face. Because of the high signal level, which is due to the short distance in the near range, it is unnecessary in this case to detect the full signal intensity.
Another advantage of the device claimed is that the electrical-capacitive properties of the detector of the measuring device are affected favorably by the shape, according to the invention, of one exemplary embodiment of the active detector face. An overly large detector surface would increase the electrical capacitance of the detector, so that the response characteristic over time, orxe2x80x94equivalent to itxe2x80x94the frequency response of the measurement system would no longer meet the necessary requirements in terms of chronological and frequency resolution of the measuring system.
In an advantageous version of the device of the invention, the face of the detector used is therefore precisely as large as the peripheral conditions sketched above require.
A simple and inexpensive embodiment of the device of the invention with the detector face claimed is obtained if the effective, that is, light-sensitive detector face is embodied by means of partially covering an originally larger detector face. To that end, for instance, a large area detector can include a layer opaque to light in those regions that are not meant to be used for detection, so that it is possible for only the claimed shape to be used as the effective, active detector face. Depending on the wavelength of the measurement signal used and on a correspondingly selected detector, the opaque regions can be created for instance applying a layer to the detector surface by vapor deposition or painting. The claimed shape for the active face of the detector could also be achieved with a simple mechanical mask or shade.
Advantageously, the device of the invention for optical distance measurement can be realized by using a laser as the light source. Lasers and in particular laser diodes are available over the entire visible spectral range of electromagnetic waves. Laser diodes, because of their compact size and by now also their high output capacities, are especially suitable for use in distance measuring devices of the claimed shape.
The optically opaque layer partly applied to the detector face can in this case for instance be a vapor-deposited metal layer, which at the desired points optically deactivates the semiconductor detector used.